This invention relates generally to apparatus for prOduCing intense magnetic fields having utility for experimentation (for example, in atomic particle acceleration and nuclear magnetic resonance) as well as for practical applications (such as magnetically levitated trains). More particularly, the invention relates to apparatus for increasing the current-carrying capabilities of superconductors and, thereby, the magnitude of the magnetic field that may be achieved.
When a magnetic field is generated by means of a helical winding or coil under non-superconductive conditions, there is ohmic resistance in the winding and heat is produced. The production of fields of greater magnitude under these conditions requires the circulation of coolants over the winding, and the costs associated with the use of such coolants and the need for replenishing them are large. In superconductivity the absence of ohmic resistance eliminates many problems associated with heat dissipation from the winding; there are, however other problems such as a limitation on the magnitude of the magnetic fields that can be safely achieved at feasible levels of cost.
A serious limitation to the widespread use of superconductivity has been the necessity to attain the extremely low temperatures required by available conductors to reach the superconductive state. In 1913 the highest critical temperature for superconductivity was approximately -269.degree. C., the boiling point of helium. Now, further search has produced materials that can be superconductive at temperatures about 100.degree. C. higher, opening the possibility for the use of much cheaper coolants such as liquid nitrogen.
Beyond the problems associated with achieving and maintaining sufficiently low temperature conditions for superconductivity however, further problems have limited the current levels and, therefore, the magnetic field that can be achieved in the central air core of a solenoid. In particular, it has been found that when the current density in a superconductor reaches a critical value, the superconductor suddenly reverts to a heat-producing, non-superconductive condition, creating the possibility of an explosion in a coil carrying a large current. This has given rise to a considerable development of the art associated with the configurations and the compositions of conductors for use as superconductors. In most cases the conductor construction is quite complex, and therefore expensive to fabricate.
It has been known for a number of years that, whereas a non-superconductive wire has a uniform current density throughout its cross section, a superconductive wire carries the current substantially entirely in the outermost shell of the cross section near, and just beneath, the surface. The current density is greatest within the cross section very near the surface, falling off essentiallY exponentially in the direction away from the surface to a penetration depth of approximately 1,000 angstrom units, or 100,000th of a centimeter, with some variation for different materials. Thus, for superconductivity the effective conductor cross section affecting the current density is much smaller than the actual conductor cross section.
In addition to a critical temperature and a critical current density there is a critical magnetic field at the surface of the superconductor itself; above this value the field-energizing winding reverts to ordinary conductivity.